It is called an EGR valve
which stands for exhaust gas recirculation. Cars in the US have had this device for quite a number of years. It allows air from the exhaust manifold to flow into the intake manifold which is supposed to reduce the amount of NOX (I think) generated during the combustion process. Over the past few years industry has started doing the same thing on steam boilers to reduce the NOX emissions. It actually has a "cooling" effect in that application and you set it up based on exit gases O2. I know that the EGR valve is controlled off of vacuum but am not sure how it knows what % open to put the opening of the valve. Someone else will have to explain that.
Race cars and vehicles in Europe do not have this device.
I would advise keeping this system in working order on the late model diesel. Just clean everything up. It will more than likely plug again.